PC troubleshooting-5V Standby power

[cyclin_al]

For now, this is the short version; I need some sleep and a chance to reflect on what I have found.

I have been troubleshooting my parents' PC, which is a Acer Aspire X3400 with AMD Athlon II processor.
After much time going through the obvious and getting more obscure, I finally have some interesting observations.

By the process of elimination, I have removed all components down to the bare minimum.
The computer is configured as motherboard, CPU with fan cooler and power supply.  Nothing else.
All RAM, peripherals and connections have been removed.

What I have noticed is that the 5V standby power of the power supply is 5 V when NOT connected to the motherboard.
When connected to the motherboard, before the computer is turned on, the standby power appears to be dragged down and measures only 3.8V.
When power is activated by briefly shorting the power switch pins on the motherboard, the computer starts and runs for 3 to 3.5 seconds.
During this brief time, the CPU fan runs.  Very interesting is that the standby power is dragged down to 0V.
The computer turns itself off after the 3 to 3.5 seconds, and the standby power momentarily reaches just over 4 V and then settles back to 3.8 V.
This is measured with a fairly cheap handheld DMM.

Does anyone have any experience with symptoms like this?  Googling while fatigued did not give any good results.
Also, any opinions on whether something this old (but not heavily used) is even worth the time and effort to chase down?

Thanks!

[cyclin_al]

As indicated earlier, sleep is needed.

A missing bit of information:
Another power supply of higher performance was tried earlier in the troubleshooting process.
The same behaviour of the computer was observed, but no voltages were monitored with the alternate power supply.

That would suggest a motherboard issue, not a PSU issue?

[SilverSolder]

Many would say, "Suspect the capacitors".  Athlon II...   that's quite an old motherboard?

If I were you, I would get your parents a nice used Dell Optiplex with an i7, for less than $200 on the bay of E's.  They perform well and are very reliable, and if something does go wrong, spare parts for those are almost free on eBay due to the popularity in the corporate world.  The best thing is that everything is 100% supported by Dell with drivers etc. so there is a "single source" for software to make it all work.

[PKTKS]

Check the  3.3 LDO which powers  (most of regular mobos)
the Super I/O chipset and  the PCI bus.

If the regulator  is not at proper 3.3V you have
a good start point.  The Super I/O layout pinout also
helps a lot finding those critical points.

Grab yourself the Super I/O datasheet

Paul

[Haenk]

Many would say, "Suspect the capacitors".  Athlon II...   that's quite an old motherboard?

If I were you, I would get your parents a nice used Dell Optiplex with an i7, for less than $200 on the bay of E's.  They perform well and are very reliable, and if something does go wrong, spare parts for those are almost free on eBay due to the popularity in the corporate world.  The best thing is that everything is 100% supported by Dell with drivers etc. so there is a "single source" for software to make it all work.

Quite commonly, there are offers for refurbished i5 including a genuine Win10 license for about 100 EUR, including taxes and shipping. Of course an i7 is much nicer, but an i5 is still a huge upgrade from that old Athlon. IMHO there is no reason (beyond educational purposes) to invest any money getting the old stuff back into running condition. And you never know, what and when the next part is failing.

[wraper]

What I have noticed is that the 5V standby power of the power supply is 5 V when NOT connected to the motherboard.
When connected to the motherboard, before the computer is turned on, the standby power appears to be dragged down and measures only 3.8V.
When power is activated by briefly shorting the power switch pins on the motherboard, the computer starts and runs for 3 to 3.5 seconds.
During this brief time, the CPU fan runs.  Very interesting is that the standby power is dragged down to 0V.
The computer turns itself off after the 3 to 3.5 seconds, and the standby power momentarily reaches just over 4 V and then settles back to 3.8 V.
This is measured with a fairly cheap handheld DMM.

I suspect you were not even measuring standby voltage. Wire of which color were you measuring?

[TheMG]

Sounds like you might be probing the PS_ON (green) wire. The PS_ON wire will have approx 5V on it when the PC is "off", but could be slightly lower (I've seen anywhere from 3-5V), the exact value really doesn't matter. When the motherboard wants the PSU to turn on, it simply pulls this signal low (0V) which signals the PSU to turn on.

The +5VSB (standby) power is on the purple/violet wire. Due to the design of how most ATX PSU operate, if the standby goes down to 0V the PSU would stop operating altogether, which from your description is not what is happening, you're seeing 0V only while the PSU is briefly actually on. Thus my belief that you might be inadvertently measuring PS_ON, not +5VSB.

For reference, since some power supply manufacturers don't necessarily follow the industry standard color code:

PS_ON is on pin 14 of a 20-pin ATX connector, or pin 16 of a 24-pin connector
+5VSB is on pin 9 of both 20 and 24 pin ATX connectors
Note that the difference in pin numbers is a result of the way the pins are numbered, they're actually in the same physical slot on both 20 and 24 pin, the 24 pin simply has 4 additional pins on one end but the rest is exactly the same.

[Sceptre]

I recommend that you measure +5VSB with a load resistor attached (at the ATX connector, standalone).  5 5W if you have it, otherwise anything up to , say, 100 .

I recently had a similar situation with a Dell Optiplex that had been given to me broken (flickering power LED, no power on).  Initially I was leaning towards a motherboard problem, since a) +5VSB was ~4.9V when disconnected from the motherboard, b) the power supply self-test button lit the green LED and started the fans, and c) a separate supply did not boot the system.  But after mulling it over for a while, I reconsidered the possibility that the +5VSB output was soft.  After all, the self test may not cover +5VSB, and the substitute supply was not known to be good.  So I connected a load to +5VSB and sure enough, it was down around 3V.  Took apart the supply and found two bulging caps (at least one of which was on the +5VSB rail).  Also realized that the issue with the other supply was a missing 4-pin auxiliary connector.  The system has been running Folding@home for the past month with a different supply until I get around to ordering the replacement caps.

Good Luck!